This invention relates to pump-off control of a beam pump driven by a pump jack.
Pump-off control sensors in beam pumped wells have been developed from downhole fluid level or pressure indicators, flow and no-flow sensors, vibration sensors, and motor current sensors to recent sensors adopting modern dynagraph card methods capable of analyzing and recording a rod load.
However, the methods of applying the sensors in related arts involve an accuracy problem and are scarcely put to practical use.
Even if the modern dynagraph card methods meet the accuracy, they require a sensor for detecting a sucker rod load, its detection signal processor, etc., and have disadvantages of complicacy and expensiveness as a result.
Since a drive motor is induction motor drive unable to adjust speed, motor stop control must be adopted as control after pump off is detected. Thus, it is feared that a pump may be stopped because of a temporary pump-off factor such as free gas in an oil well, resulting in lowering of the production amount in the oil cell.
To avoid this, so-called on-off operation control of stopping a pump motor when pump off is detected three to five or more successive times and again starting the pump after the expiration of a given time has been adopted.
However, this method places excessive mechanical and electrical stresses on the pump unit and the motor by the on-off operation and has disadvantages of fastening wear of facilities and increasing maintenance costs.
The invention provides the means for solving problems as described above and a pump off control method according to a first embodiment of the invention is as claimed in claims 1 to 4.
According to the pump off control method, without using an expensive dynagraph card system in the related art made up of a rod load sensor and a microcomputer, pump off control software is built in an inverter used for speed control of a pump jack, whereby pump off can be detected not only at low cost, but also precisely.
In addition, since the pump jack speed is controlled, as pump off is detected, the pump jack speed can be lowered to a state in which no pump off exists, whereby continuous production in an oil well can be executed without imposing excessive load on a downhole pump or a sucker rod system. That is, the effects of enhancing the productivity in an oil and improving safety of the facilities can be produced as compared with an oil well to which the pump jack in the related art driven at constant speed is applied.
The maximum speed of the downhole pump can be preset corresponding to change in oil well circumstances accompanying the comparatively long time passage such as an increase in free gas or lowering of the oil well level, so that it is made possible to lower the possibility that pump off will occur, contributing to stable operation in an oil well accordingly.
A pump off control method according to a second embodiment of the invention is as claimed in claims 5 to 8.
That is, the speed of an induction motor for driving a pump jack can be controlled by an inverter of a variable voltage variable frequency power supply and means for detecting the speed of the motor and the instantaneous value of the secondary current of the motor, means for detecting the elapsed time from the reference point at which the secondary current instantaneous value reaches the maximum value, which will be referred to as secondary current maximum value time in the invention, and each down stroke time of the pump jack, means for detecting and storing each secondary current maximum value time, and means for setting reference of the elapsed time at which the secondary current instantaneous value from the reference point reaches the maximum value, which will be referred to as setup reference time in the invention, for comparison with the detected storage value are placed.
When the secondary current maximum value time becomes longer than the setup reference time, it is detected as occurrence of a pump off condition and if pump off occurs, the motor speed is lowered gradually in sequence. In contrast, when the secondary current maximum value time is shorter than or equal to the setup reference time, it is detected as reset of pump off and the motor speed is controlled so as to recover the lowered speed in sequence, whereby overpressuring of the downhole pump is prevented and high production of crude oil is enabled in response to the circumstances of an oil well.
According to the invention, without using an expensive dynagraph card system made up of a rod load sensor and a microcomputer, pump off control software is incorporated in a vector control inverter used for speed control of a pump jack, thus pump off can be detected not only at low cost, but also precisely for the reason described later. In addition, since the pump jack speed is controlled, as pump off is detected, the pump jack speed can be lowered to a state in which no pump off exists, so that continuous production in an oil well can be executed without imposing excessive load on a downhole pump or a sucker rod system.
That is, the effects of enhancing the productivity in an oil and improving safety of the facilities can be produced as compared with an oil well to which the pump jack in the related art driven at constant speed is applied. The maximum speed of the downhole pump can be preset corresponding to change in oil well circumstances accompanying the comparatively long time passage such as an increase in free gas or lowering of the oil well level, so that it is made possible to lower the possibility that pump off will occur, contributing to stable operation in an oil well accordingly.
As compared with the described method of calculating the effective value or the average value of the secondary current instantaneous value of the motor in down stroke, comparing the value with the reference value, and detecting pump off in the first embodiment, it is not necessary to change the setup reference value as the pump jack speed is changed.
The method, which is based on detection of the differenced between the secondary current maximum value time and the setup reference time, is not related to the magnitude of the secondary current and thus has an excellent feature that it is hard to be affected by variations in the downhole pump load caused by change in the content of water and impurities in crude oil, and it is made possible to precisely detect pump off accordingly.
In addition, operation processing becomes simple as compared with the method of the first embodiment, thus the method of the second embodiment has the advantage that a controller can be configured easily.